Logothetis N K, Pauls J, Poggio T
Division of Neuroscience, Baylor College of Medicine, Houston, Texas 77030, USA.
Curr Biol. 1995 May 1;5(5):552-63. doi: 10.1016/s0960-9822(95)00108-4.
The inferior temporal cortex (IT) of the monkey has long been known to play an essential role in visual object recognition. Damage to this area results in severe deficits in perceptual learning and object recognition, without significantly affecting basic visual capacities. Consistent with these ablation studies is the discovery of IT neurons that respond to complex two-dimensional visual patterns, or objects such as faces or body parts. What is the role of these neurons in object recognition? Is such a complex configurational selectivity specific to biologically meaningful objects, or does it develop as a result of extensive exposure to any objects whose identification relies on subtle shape differences? If so, would IT neurons respond selectively to recently learned views of features of novel objects? The present study addresses this question by using combined psychophysical and electrophysiological experiments, in which monkeys learned to classify and recognize computer-generated three-dimensional objects.
A population of IT neurons was found that responded selectively to views of previously unfamiliar objects. The cells discharged maximally to one view of an object, and their response declined gradually as the object was rotated away from this preferred view. No selective responses were ever encountered for views that the animal systematically failed to recognize. Most neurons also exhibited orientation-dependent responses during view-plane rotations. Some neurons were found to be tuned around two views of the same object, and a very small number of cells responded in a view-invariant manner. For the five different objects that were used extensively during the training of the animals, and for which behavioral performance became view-independent, multiple cells were found that were tuned around different views of the same object. A number of view-selective units showed response invariance for changes in the size of the object or the position of its image within the parafovea.
Our results suggest that IT neurons can develop a complex receptive field organization as a consequence of extensive training in the discrimination and recognition of objects. None of these objects had any prior meaning for the animal, nor did they resemble anything familiar in the monkey's environment. Simple geometric features did not appear to account for the neurons' selective responses. These findings support the idea that a population of neurons--each tuned to a different object aspect, and each showing a certain degree of invariance to image transformations--may, as an ensemble, encode at least some types of complex three-dimensional objects. In such a system, several neurons may be active for any given vantage point, with a single unit acting like a blurred template for a limited neighborhood of a single view.
长期以来，人们一直认为猴子的颞下皮质（IT）在视觉物体识别中起着至关重要的作用。该区域受损会导致感知学习和物体识别方面的严重缺陷，而不会显著影响基本视觉能力。与这些切除研究一致的是，发现了对复杂二维视觉模式或诸如面部或身体部位等物体做出反应的IT神经元。这些神经元在物体识别中起什么作用？这种复杂的构型选择性是否特定于具有生物学意义的物体，还是由于广泛接触任何其识别依赖于细微形状差异的物体而发展形成的？如果是这样，IT神经元是否会对新物体特征的最近学习视图做出选择性反应？本研究通过结合心理物理学和电生理学实验来解决这个问题，在这些实验中，猴子学会对计算机生成的三维物体进行分类和识别。
发现一群IT神经元对以前不熟悉的物体视图有选择性反应。这些细胞对物体的一个视图放电最大，并且随着物体从这个偏好视图旋转离开，它们的反应逐渐下降。对于动物系统无法识别的视图，从未遇到过选择性反应。在视图平面旋转期间，大多数神经元也表现出方向依赖性反应。发现一些神经元围绕同一物体的两个视图进行调谐，并且极少数细胞以视图不变的方式做出反应。对于在动物训练期间广泛使用的五个不同物体，并且其行为表现变得与视图无关，发现多个细胞围绕同一物体的不同视图进行调谐。许多视图选择性单元对物体大小或其图像在中央凹旁的位置变化表现出反应不变性。
我们的结果表明，由于在物体辨别和识别方面的广泛训练，IT神经元可以形成复杂的感受野组织。这些物体对动物来说都没有任何先前的意义，它们也与猴子环境中熟悉的任何东西都不相似。简单的几何特征似乎无法解释神经元的选择性反应。这些发现支持这样一种观点，即一群神经元——每个神经元都调谐到不同的物体方面，并且每个神经元对图像变换都表现出一定程度的不变性——作为一个整体，可能至少对某些类型的复杂三维物体进行编码。在这样一个系统中，对于任何给定的有利位置，可能有几个神经元是活跃的，单个单元就像单个视图的有限邻域的模糊模板。
